The present invention relates to a burner for heating furnaces, in particular closed furnace chambers, or for heating interior spaces of jet pipes by flameless oxidation (FLOX®).
For preheating the combustion air of burners either recuperators or regenerators are used. Recuperators are heat exchangers that utilize exhaust gas heat for preheating combustion air, whereby heat of the hot exhaust gas is transferred through a dividing wall to the combustion air. In contrast, regenerators are heat storage devices through which alternately exhaust gas and combustion air is conducted whereby the storage device heat up in one phase and, as a result, the exhaust gas is cooled. In order to transfer the absorbed heat to the combustion air, the combustion is subsequently conducted through hot storage device in another phase. Regenerator technology offers a higher degree of heat recovery with a given construction volume, however, also requires considerable expense and effort for the periodic switching from exhaust gas mode to fresh air mode. In the past, regenerator burners were preferably used with relatively high efficiency. They are also increasingly in demand for burners with medium efficiency ranges.
DE 44 20 477 A1 discloses industrial burners with regenerative air preheating. Such an industrial burner is suitable for the medium power range of 50 to 300 kW. It comprises regenerative cartridges which are located in the furnace wall, so that the temperature of the exhaust gas is already reduced in the furnace wall. Nozzles are arranged on the hot side of the regenerator cartridges, said nozzles discharging the preheated air at a high speed. As a result of this, a strong exhaust gas recirculation is produced in the furnace. The thusly effected flameless oxidation (FLOX® principle) is particularly suitable when the air is preheated to high temperatures in order to avoid the thermal NOx formation and to improve the uniformity of temperature in the furnace. Switching valves for switching the individual regenerator cartridges from heat absorption to heat release are located on the burner head.
For many years, such burners have been successfully used for the direct heating of furnaces. With a furnace temperature of e.g., 1100° C., it is possible to preheat the combustion air to 950° C. When natural gas is used as fuel, this provides for a combustion efficiency of 85%. However, the exhaust gas is not always completely discharged via the regenerators. A certain partial stream is directed into other regions such as, for example, a preheating zone of a flow-through furnace in order to preheat the material that is to be treated.
It has also been known to heat furnace spaces indirectly by the use of jet pipes. A jet pipe encloses an interior space that is heated by a burner whereby the jet pipe is heated to a temperature that is high enough to heat the furnace space using the radiation heat emitted by the jet pipe. Considering these jet pipes, the NOx problem becomes prevalent because the temperature inside the jet pipe is higher than in the furnace. In addition, the recirculation required for maintaining the flameless oxidation is impaired because of the limited open flow cross-section. However, in order to still achieve the desired recirculation and flameless oxidation, the pulse rate of the air should be increased at the output nozzles which direct the stream of combustion air into the interior of the jet pipe.
As a rule, jet pipes are operated at an internal pressure that approximately corresponds to the external pressure. Therefore, the partial exhaust gas stream that is to be taken from the jet pipe is removed by an exhaust gas blower drawing the gas through the regenerator arrangement. Due to the flow resistance of the regenerators, and due to the high flow resistance of the air nozzles on account of the high flow rate, and due to the resultant pressure gradient, the exhaust gas blower must overcome a high pressure difference. The loss of pressure in the burner thus increases with the square of the flow rate in the air nozzles.
It is the principle object of the present invention to overcome the mentioned technical problems with the application of flameless oxidation for heating closed or enclosed spaces. In particular, the blower power requirements should be as low as possible and the heating efficiency should be as high as possible.